Field of the Invention
The present invention relates to an image forming device, a control method for the image forming device, and a control program for the image forming device, and particularly relates to an image forming device that forms an image on a paper by electrophotography, a control method for the image forming device, and a control program for the image forming device.
Description of the Related Art
An electrophotographic image forming device is widely used for such purposes as a multi-function peripheral (MFP) having a scanner function, a facsimile function, a copy function, a function as a printer, a data communication function, and a server function, a facsimile machine, a copy machine, and a printer.
In the electrophotographic image forming device, a two-component development system is widely used. In the two-component development system, toner and developer including a magnetic carrier is used. In the case where image forming is performed by the two-component development system, malfunction may occur when the toner or the carrier grossly adheres to a portion immediately before an image area on an image carrying body. To avoid such malfunction, normally, timing to start applying charging bias voltage and timing to start applying developing bias voltage are set.
Typical examples of such malfunction caused by the carrier adhering to the portion immediately before the image area on the image carrying body (hereinafter may be referred to as carrier adhesion) may be as follows. More specifically, a flaw may be made on a cleaning blade when the carrier remaining on the image carrying body is scraped from the image carrying body by the cleaning blade. When the flaw is made on the cleaning blade, cleaning failure may occur on the image carrying body later due to the portion having the flaw, and a black stripe may appear on a formed image.
Furthermore, in a transfer system in which a transfer member like a transfer roller contacts the image carrying body, a carrier adhering to the image carrying body may adhere to the transfer member. Carrier adhesion to the transfer member may cause transfer failure at the time of transferring a toner image to a transfer material (such as paper and an intermediate transfer belt) from the image carrying body next time. For example, an abnormal state such as a white spot appearing on a formed image may be caused.
On the other hand, typical examples of such malfunctions caused by toner adhering to the portion immediately before the image area on the image carrying body (hereinafter may be referred to as toner adhesion) are as follows. More specifically, in the case of performing transfer by a contact transfer system, the toner on the image carrying body may directly adhere on the transfer member. When the toner adheres to the transfer member, an abnormality such as marking back on a transfer material may be caused at the time of transferring a toner image to the transfer material next time.
Particularly, when a transfer roller made of a foamed sponge having an uneven surface is used as the transfer member, the problem of toner adhesion is apparent. In this case, the toner adhering to the transfer roller enters a recessed portion of the transfer roller. The toner having entered the recessed portion adheres to a surface of the transfer material contacting the transfer roller when the transfer material passes. Since the transfer roller contacts the image carrying body in a pressurized manner, a diameter of the transfer roller in a portion contacting the image carrying body is different from a diameter thereof in a portion not contacting the image carrying body. On the surface of the transfer roller contacting the image carrying body, a shape of the recessed portion is changed and the toner is discharged from the recessed portion This may cause the discharged toner to adhere to a back surface of the transfer material, and a stain may be made with the toner.
Charge of the carrier has a sign opposite to a sign of charge of the toner; one is positive and the other is negative. Therefore, prevention of carrier adhesion and prevention of toner adhesion by controlling a potential difference between the developer carrying body and the image carrying body mutually have a trade-off relationship. The malfunction caused by carrier adhesion gives more damage on an image forming device. Therefore, generally, the potential difference between the developer carrying body and the image carrying body is controlled so as to prevent carrier adhesion completely first and then suppress toner adhesion as much as possible.
Meanwhile, JP 54-12843 A discloses an electrophotographic device in which a start time of development in a developing device is delayed until a non-charged portion of a photoreceptor reaches a developing position. This prevents wasteful toner consumption.
JP 2001-265193 A discloses an image forming device in which timing to start applying charging bias voltage and developing bias voltage is changed in accordance with a detection value of a toner concentration detection sensor. The toner concentration detection sensor is a sensor to detect a ratio between toner and carriers inside the developing device.
In an image forming device described above, an occurring state of carrier adhesion is varied by a charging state on a surface of an image carrying body. Therefore, when design is made so as not to cause carrier adhesion under any kinds of conditions, there may be a problem that a toner adhesion amount is increased.
The occurring states of carrier adhesion and toner adhesion are varied by a charging range on the surface of the image carrying body. Normally, potential on the surface of the image carrying body is set so as to keep a constant potential difference relative to bias voltage applied to a developing roller. Therefore, the occurring states of carrier adhesion and toner adhesion are substantially the same between cases where the potential on the surface of the image carrying body has a large absolute value and a small absolute value. On the other hand, comparing a case of having a wide charging range on the surface of the image carrying body with a case of not having such a wide charging range, an area charged when application of charging bias voltage is started reaches a developing nip portion in early timing. Therefore, in the case of having the wide charging range, carrier adhesion may be caused unless application of the developing bias voltage is started in earlier timing than the case of not having such a wide charging range. In contrast, when the timing to start applying the developing bias voltage is thus set early conform to the case of having the wide charging range, there may be a problem in which a toner consumption amount is increased in the case of having a narrow charging range.
These problems are apparent especially in an image forming device in which the surface of the image carrying body is charged by a discharging phenomenon, such as a roller charging system in which direct current bias voltage (DC bias voltage) is applied as the charging bias voltage. The reason is that a range to be charged is largely varied by the way of discharging. More specifically, when design is made such that application of the developing bias voltage is started early conforming to a case of performing relatively intense discharge in order to prevent carrier adhesion, a toner consumption amount may be increased in the case of performing relatively weak discharge.